1. Field of the Invention
The present invention relates to a regulator in which a switching regulator and a series regulator are inter-connected, and more particularly to a "hybrid" regulator configured to utilize the advantage of a series regulator, namely, a superior regulation performance having no ripple even when a variation in load occurs, and to utilize the advantage of a switching regulator, namely, high efficiency.
2. Description of the Prior Art
In accordance with the advent of Green Round, many efforts to reduce the absolute quantity of energy used have recently been made in a variety of technical fields. Such efforts have also been made in conjunction with electronic and electrical appliances. In addition to such efforts, another effort has been made to increase the efficiency of energy used, thereby minimizing a loss of energy.
Meanwhile, all electronic appliances, electrical appliances, electric home appliances and a variety of industrial electronic appliances, which are currently used, essentially require a stable power supply, namely, DC--DC converter. Most appliances, in which electronic circuits are included, use a stable DC power such as +5V DC, +12V DC or +15V DC in general.
For electronic devices such as IC's, transistors, lamps, etc., a maximum allowable voltage is set. When an electronic device is supplied with the voltage greater than its maximum allowable voltage, it may be damaged or reduced in use life. Where an operational amplifier or comparator is used to amplify signals having a low amplitude or to compare such signals, a variation in voltage occurring in an external power supply adapted to supply voltage to the circuit may cause the malfunction of the circuit, thereby resulting in a degradation in the accuracy or stability. In addition to developments of electronic devices having high accuracy, therefore, developments of stable power supply devices are also important.
Generally, a regulator is a device for maintaining output voltage or current in a strong and uniform state irrespective of a variation in input or output load. Regulators currently used are mainly classified into switching regulators and series regulators. The series regulator is generally used when the good regulation performance without ripple is needed. On the other hand, the switching regulator is used to obtain high efficiency while reducing in size.
Referring to FIG. 1, an example of a series regulator is illustrated. The series regulator is also called a "linear regulator" or "dropper regulator". This regulator has an advantage of an excellent output-voltage regulation and a disadvantage of a poor power transformation efficiency. In this regard, the series regulator is suitable for the case in which a prominent regulation but low electric power is required. Since such a series regulator is controlled in a voltage series feedback scheme while having no delay element (for example, an inductor connected in series to the regulator, or a capacitor connected in parallel to the regulator) arranged on its main power supply path, it inherently has a superior regulation performance in steady state as well as transient state condition.
In the series regulator shown in FIG. 1, a "differential voltage" between an external supply voltage Vdd and an output voltage Vo observed in load resistor R4 is applied between the collector and emitter of an output transistor Q1. In this state, the same amount of current as that required for the load resistor R4 is supplied to the emitter of the output transistor Q1 via the collector of the output transistor Q1. For this reason, this series regulator exhibits a poor power efficiency.
In this case, the power used in the load resistor R4 is expressed by the following expression (1) whereas the power loss in the output transistor Q1 is expressed by the following expression (2): EQU P.sub.R4 =V.sub.R4 .times.I.sub.R4 [Expression 1] EQU P.sub.Q1 =V.sub.CE .times.I.sub.C .congruent.V.sub.CE .times.I.sub.R4[Expression 2]
In order to reduce the power loss in the transistor Q1, expressed by Expression (2), it is required to decrease the voltage V.sub.CE applied between the collector and emitter of the output transistor Q1 or reduce the current IC flowing through the collector of the output transistor Q1, or to simultaneously decrease the voltage V.sub.CE and current I.sub.C.
The current I.sub.R4 flowing through the load resistor R4 is almost the same as the collector current I.sub.C. The sum of the voltage V.sub.R4 applied across the load resistor R4 and the collector-emitter voltage V.sub.CE is the same as the external supply voltage Vdd. Assuming that the loss of power in other elements of the series regulator is ignored, accordingly, the power efficiency of the series regulator is approximately expressed by the following expression (3): ##EQU1##
In Expression (3), ".eta." represents a power efficiency, and "P.sub.total " represents the totally consumed power of the said series regulator.
Where the series regulator is adopted to regulate the voltage of +5V for driving a TTL IC when an external supply voltage Vdd of +12V is used, +7V DC, which is a differential voltage between the external supply voltage and output voltage, is applied between the collector and emitter of the output transistor Q1. In this case, accordingly, the power efficiency of the series regulator corresponds to about 42%.
Of course, the power transformation efficiency may be enhanced by increasing the voltage V.sub.R4 while decreasing the voltage Vdd, as apparent from Expression (3). However there is a limitation in optional setting of the power efficiency because the range, in which a desired external supply voltage or desired output voltage is selected, is limitative.
On the other hand, the loss of power consumed during the power transformation is completely changed into heat. Therefore, a large heat sink should be additionally used in order to prevent the output transistor Q1 from being heated to a temperature higher than an allowable temperature. This results in a bulky volume. For this reason, it is difficult to use the series regulator as a power supply in the case in which a high power of more than 20 W should be used.
Referring to FIG. 2, an example of a switching regulator is illustrated. As shown in FIG. 2, the switching regulator has a configuration similar to the series regulator, except that it uses a comparator U2 as its control element whereas the series regulator uses an operational amplifier U1 as its control element. The switching regulator also includes a regulation circuit, composed of inductor and capacitor, arranged between the output transistor Q1 and load resistor R4, different from the series regulator. In other words, the switching regulator carries out a switching control whereas the series regulator carries out a linear control. Accordingly, the switching regulator involves a switching ripple, even though there is no output ripple involved in the series regulator.
In the switching regulator of FIG. 2, output voltage applied across the resistor R4 is sensed by negative feedback resistors R2 and R3. For the output voltage, a comparison is then carried out in the comparator U2. Based on the result of the comparison, the comparator U2 outputs a signal of high or low level. In response to the output signal from the comparator U2, the output transistor Q1 performs an ON or OFF switching operation. As a result, voltage of a high level (namely, Vdd) or low level (namely, zero) is applied to an inductor L1. In a steady state, the pulse waveform of the voltage is regulated by the regulation circuit which includes a capacitor C1 along with the inductor L1. The output voltage across the capacitor C1 has a value corresponding to an average value of a pulse wave applied to inductor L1, so that it has a waveform involving a switching ripple.
A ripple involved in an output voltage of the switching regulator includes a switching ripple caused by a switching operation itself and a load variation ripple due to a variation in load. The switching ripple can be reduced by increasing the switching frequency. In this case, however, the loss of power caused by the switching operation increases with proportion to switching frequency. As a result, a degradation in the power efficiency inevitably occurs. For this reason, it is necessary to use elements having a high operating speed, too. However, this results in an increase in the manufacturing costs.
The load variation ripple can be reduced by using a regulation circuit having large inductance and capacitance, thereby improving the regulation performance. In this case, however, the inductor and capacitor used are bulky. An increase in the manufacturing costs also occurs.
As apparent from the above description, the switching regulator meets the purpose of Green Round in that it has the advantages of reduction of power loss, namely, high power efficiency, and a reduced size. However, this switching regulator has disadvantages in that a switching ripple is involved in its output voltage and its ability to cope with a variation in load is insufficient.
The following Table 1 shows the advantages and disadvantages of existing series regulators and switching regulators which are opposite to each other.
TABLE 1 ______________________________________ Advantages Disadvantages ______________________________________ Series Regulator Excellent Regulation Performance Poor Power Involving No Ripple Efficiency Strong against Variation in Load Bulky Heat Sink Switching Regulator Good Power Bad Regulation Performance Efficiency Involving Switching Ripple Compact Heat Sink Weak against Variation in Load ______________________________________